Owlstone Inc.

Detecting H2S Scavengers and their Byproducts with

Lonestar

Owlstone Whitepaper

Alasdair Edge alasdair.edge@owlstone.co.uk

006012 v 1.0

Owlstone’s Lonestar Analyzer provides an accurate, portable and easy-to-use means for non-specialist personnel to carry out real-time, at-line monitoring of H2S scavengers and their reaction by-products.

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Detecting H2S Scavengers and their Byproducts

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Hydrogen Sulfide in Crude: A Problematic Contaminant

Hydrogen sulfide (H2S) is a poisonous, corrosive contaminant found in unrefined crude oil and natural gas.

It may also be produced by heating sulfur-containing compounds in oil during the refining process. H2S is

highly toxic, with 1000 ppm most likely fatal within a few inhalations, and an acceptable ceiling

concentration of just 20 ppm given by the Occupational Safety and Health Administration (OSHA Report).

Although it has a strong and characteristic odor (of ‘rotten eggs’) at low concentrations, at levels above

about 100 ppm it can paralyze the olfactory nerve, dulling the sense of smell and meaning operators may

be unaware of the presence of potentially dangerous amounts of the gas. It is heavier than air, so will tend

to collect at the bottom of confined spaces.

H2S in crude oil and natural gas can also cause damage to pipework, either by reacting directly with steel to

create an iron sulfide corrosion film, or by increasing the acidity of the liquid/gas mixture in the pipes (Sun

and Nesic 2007). Furthermore, when dissolved in water, H2S may be oxidized to form elemental sulfur. This

can also produce an iron sulfide corrosion film when in direct contact with the metal surface (Fang, Young

and Nesic 2008).

The average sulfur content (and by implication the H2S content) of crude oil input to US refineries has been

on an upward trend over the last 20 years (EIA report), while permissible emissions limits have been

tending downwards (EPA Standards of Performance for Petroleum Refineries).

For all of these reasons, it is essential to remove H2S from crude oil as quickly and efficiently as possible. To

do so, many companies utilize chemical compounds known as ‘scavengers’, which remove H2S in a process

sometimes known as ‘stripping’.

What Kinds of Scavengers Exist?

There are two broad classes: regenerative and non-regenerative, which undergo reversible and non-

reversible reactions with H2S respectively. Regenerative include alkanolamines (MEA, DEA, MDEA and

DGA). May also include oxidizers such as peroxide and caustic (NaOH or NaOH/KOH). Non-regenerative

include metal oxides (e.g. Fe2O3 or ZnO), oxidizing chemicals such as KMnO4 or K2Cr2O7, aldehydes, metal

carboxylates and chelates, and the most widely-used, triazines (Madsen 2011). Of these, it is 1,3,5-tri-(2-

hydroxyethyl)-hexahydro-s-triazine (HHTT, CAS number 4719-04-4) that is primarily used in oil refining,

where it is usually simply referred to as triazine.

How Does the HHTT Scavenging Process Work?

The HHTT molecule consists of a central s-triazine ring, with a 2-hydroxyethyl group attached to each

nitrogen. In the scavenging reaction, the sulfur replaces the nitrogen and 2-hydroxyethyl, which then

combine with the hydrogen from the H2S to form monoethanolamine (MEA). In principle, all three

nitrogens could be replaced, to leave trithiane (C3H6S3) and three MEA molecules. Under normal refinery

conditions, though, only two sulfurs displace nitrogen from the HHTT, forming dithiazine (5-(2-

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Detecting H2S Scavengers and their Byproducts

with Lonestar

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hydroxyethyl)hexahydro-1,3,5-dithiazine) and two MEA molecules. Pictorially, the reaction proceeds as

follows:

Figure 1 - Triazine Scavenging Reaction

What Are the Effects of Excess Scavengers?

HHTT scavengers are effective in removing H2S from crude oil, but are not themselves free of problems.

Refineries have reported fouling of systems and solid deposits caused by crude oil treated with HHTT

scavengers. It is believed (Taylor and Matherly 2011, Taylor et al 2012) that this is due to the formation of a

polymer of dithiazine linked by long carbon and sulfur chains. This material is also known as amorphous

dithiazine, and appears to begin forming when HHTT is around 60% spent. In order to prevent the

formation of amorphous dithiazine, levels of HHTT, MEA and dithiazine must be closely monitored.

Owlstone’s Lonestar Analyzer provides a rapid, portable and easy-to-use way of doing so.

How Are Scavengers Detected and Scavenger Levels Monitored?

The Lonestar system provides chemical identification and quantification using field asymmetric ion mobility spectrometry (FAIMS). The FAIMS process comprises three phases: ionization, separation and measurement.

In the first phase, vapor from a volatile sample is ionized using a 63Ni ionizer. The ions then pass into a channel, where they are exposed to an electric field perpendicular to their direction of travel, created by an alternating voltage of the form shown in Figure 2.

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Detecting H2S Scavengers and their Byproducts

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Figure 2 – Alternating Square Wave

This voltage causes the ions to travel in a saw-tooth trajectory, as in Figure 3. The product of the voltage and the time for the short-duration, high positive voltage and long-duration, low negative voltage sections are the same. This means that if the ion’s mobility, k (where v=kE, v is the velocity of ion travel and E is the electric field strength) is the same in the high and low electric fields, then it will travel through the channel along the path marked 1 in Figure 3. In general, though, ions’ mobility changes with field strength. Ions whose mobility increases with field strength will travel along a path like that marked 2, and hit one side of the filter, while those whose mobility decreases will travel along a path like that marked 3, and hit the other side of the filter. Measuring the ion current leaving the filter allows us to tell many ions travelled along path 1, allowing us to work out the concentration of those ions in the original mixture.

Figure 3 – Ion trajectories in FAIMS

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Detecting H2S Scavengers and their Byproducts

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All rights reserved © 2013. Owlstone Inc.

If we wish to change which ions pass through the filter, we can apply an additional DC compensation

voltage (CV) across the channel, which will result in an additional drift velocity towards one of the sides.

The strength of the compensation voltage necessary to allow a particular ion to pass through the filter

depends on the difference between its high- and low-field mobilities, and this is how FAIMS can be used to

test for particular substances. By applying the compensation voltage for the substance we are interested in,

and then measuring whether an ion current is produced at the end of the channel, we can test whether

that substance is present.

HHTT, MEA and dithiazine all pass through the system at different compensation voltages, so the amounts

of each present in a particular crude oil sample can be determined by measuring the ion currents at their

respective compensation voltages. Calibration for each compound is carried out by measuring the ion

current produced by a range of samples of known concentrations, and then interpolating the results.

Experimental Setup and Sample Preparation

FAIMS analysis is very simple to carry out using the Lonestar. Crude oil is placed into a glass vial, which is

then inserted into a sample holder and attached to the system. The air flow through the system is opened,

to ensure the sample vapor enters the ion channels, and analysis is carried out at the push of a button. The

whole process can be seen in action here.

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Detecting H2S Scavengers and their Byproducts

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Figure 4 Lonestar photograph and experimental schematic

Owlstone Results: Detection of Scavengers in Crude

Figure 5 shows the ion current recorded as the compensation voltage is swept from -6 to +6V. The peak due

to triazine (HHTT), seen at around -0.4V, is clearly separated from the peak due to the crude oil

background, at around 0.8V. This shows that detection of HHTT in crude can be easily achieved.

Figure 5 - Triazine peak for desalter crude

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Detecting H2S Scavengers and their Byproducts

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Figure 6 shows the calibration curve for triazine (HHTT) in crude oil. The limit of detection for the system is

taken to be 3x the standard deviation of the background noise, which in this case is equivalent to

approximately 0.2 A.U. Thus, triazine can be detected down to 3ppm with this setup.

Figure 6 - Calibration curve for Eagle Ford crude, showing detection of HHTT down to 3ppm.

Owlstone Results: Detection of Scavengers and By-Products in Water

Figure 7 shows the results obtained from a commercially available HHTT scavenger sample diluted 1000

times with water, which was then spiked with known concentrations (25ppm) of MEA and dithiazine. The

clear separation between the labeled triazine (HHTT), dithiazine and MEA peaks shows that the three

compounds can be simultaneously detected and quantified.

Figure 7 - Scavenger with 25 ppm of dithiazine and MEA added, showing capacity for simultaneous measurement.

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Detecting H2S Scavengers and their Byproducts

with Lonestar

All rights reserved © 2013. Owlstone Inc.

Figure 8 is a calibration curve for dithiazine, showing detection and quantification down to 25ppm.

Figure 8 – Calibration curve of dithiazine in water

Conclusion

By detecting and quantifying (HHTT) triazine and MEA in crude oil, and triazine, MEA and dithiazine in

water, Lonestar provides the means to monitor the entire scavenging process and prevent potentially

expensive fouling by amorphous dithiazine.

y = 0.012x + 0.032R² = 0.991

0

0.5

1

1.5

2

2.5

3

0 50 100 150 200 250

Dit

hia

zin

e Io

n C

ou

nt

(A.U

.)

Dithiazine Concentration (ppm)

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Detecting H2S Scavengers and their Byproducts

with Lonestar

All rights reserved © 2013. Owlstone Inc.

Lonestar Specifications

Technology Field Asymmetric Ion Mobility Spectrometry (FAIMS)

Analytes Triazine (CAS: 4719-04-4)

Monoethanolamine (CAS: 141-43-5)

Dithiazine

Dynamic range 3-600ppm

Accuracy +/- 5% of maximum concentration (+/-30ppm)

Analysis time <10 min

Calibration frequency 6 months

Features of Lonestar H2S Scavenger Analyzer

Selective detection of H2S scavengers and reaction products in range of crude oils

Quantitation of scavenger between 3 and 600ppm

Ease of operation - No special sample preparation or extraction required, operated by

non-specialist.

Analysis time under 10 minutes

Benefits of Deploying Lonestar H2S Scavenger Analyzer

Petroleum processors can identify impurities on the spot, before custody transfer

Avoid corrosion and fouling in downstream equipment